These experiments are part of a series of experiments that are designed to determine the structural plan of a somatosensory projection nucleus--the thalamic n. ventralis posterolateralis (VPL) in the rat. The VPL of the rat has a relatively simple structural plan with few inputs, no serial or axoaxonic synapses and no interneurons. Therefore, a more complete analysis of the structure of VPL is possible in the rat than in more complicated areas and a model can be formulated of an elementary relay nucleus. The present study centers on two aspects of the somatosensory circuit related to the processing of information from the cutaneous receptors. 1. What is the structural organization of the input/output circuits in VPL as they relate to functional somatotopy/ and 2. How is the thalamic reticular nucleus (TRN) related to the input/output circuits in VPL? Such questions of connectivity must be resolved in order to form hypotheses on how sensory information is utilized by the nervous system. The first question can be addressed by small injections of a marker compound (HRP) into the dorsal column nuclei to show the distribution of DCN axons within VPL, and by injections of HRP into clusters of VPL neurons. The antegrade transport of HRP will show the distribution of VPL axons within SI and SII cortex and into the TRN. The retrograde transport of HRP will show the distribution of neurons projecting to VPL from the DCN, TRN the SI and SII cortex as well as other thalamic nuclei that are suspected of projecting to VPL. In each case the injection of HRP will be into a site that has been somatotopically specified by its receptive field. Because there are no interneurons in VPL, an important source of inhibition may come from TRN, yet there has not been a definitive study of the synaptology of TRN. It is proposed here to study the normal TRN and also TRN following lesions of somatosensory cortex and of VPL in order to identify the synapses in TRN which originate from those sources. This will be accomplished by quantitative electron microscopy. Experiments utilizing both the injection of HRP into one source of synapses to TRN neurons e.g., VPL clusters and lesions of another source, e.g., somatosensory cortex, will show the relationship of TRN to the input/output circuits of the VPL clusters.